The present invention relates to improvements in a process for producing large-sized steel pipes of rectangular or square configuration for use in buildings and other structures.
Such a large-sized steel pipe having a rectangular or square configuration (hereinafter referred to simply as "square steel pipe") is usually produced by bending two steel plates into U-shaped sections respectively and joining the bent plates by butt welding. Since the steel pipe thus produced has two weld lines, the welding material and operation account for a substantial portion of the cost of the pipe, and removal of the strain resulting from the welding operation is similarly costly. These factors present difficulties in reducing the manufacturing cost of steel pipes. Whereas steel pipes of small size can be produced by bending a steel plate and welding the plate along a single line, production of large-sized steel pipes, 350 mm.times.350 mm or larger in cross section, usually involves many difficulties in the bending of the material and in the other processing steps, so that it has heretofore been considered impractical to produce large-sized steel pipes from a single plate.
Furthermore, the conventional process comprises many steps such as edge preparation and bending of two steel plates, fitting, tack welding and welding of the bent plates, and removal of strain from the welded product. Accordingly the process is difficult to practice by a continuous operation, requires labor for the transfer of the workpieces from step to step and consequently renders the product relatively costly.
To overcome these problems, applicant has already proposed a process for producing large-sized square steel pipes from a single steel plate, but the proposed process still involves the following problem. Since the steel plate is cut to predetermined lengths and each length of plate is separately formed into the rectangular or square shape prior to welding the welding phase of the process is repeatedly interrupted. This results in wasted time and greater welding time. The workpiece must be cut to an altered length every time the length of the steel pipe to be produced is altered. This requires an interruption of the processing operation. Moreover, when the workpiece is welded by submerged welding, the workpiece needs edge preparation and, when shaped into a tubular form, must also be welded from inside. Thus the prior process still remains to be fully simplified.
Reference is made to aplicant's co-pending U.S. application Ser. No. 48,535, filed June 13, 1979.